Object detection method and apparatus

ABSTRACT

Method and apparatus for detecting objects. In one embodiment, a person entering a secured zone is illuminated with low-power polarized radio waves. Differently polarized waves which are reflected back from the person are collected. Concealed weapons are detected by measuring various parameters of the reflected signals and then calculating various selected differences between them. These differences create patterns when plotted as a function of time. Preferably a trained neural network pattern recognition program is then used to evaluate these patterns and autonomously render a decision on the presence of a weapon. An interrupted continuous wave system may be employed. Multiple units may be used to detect various azimuthal angles and to improve accuracy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing of U.S. ProvisionalPatent Application Ser. No. 60/680,627, entitled “Object DetectionMethod and Apparatus”, filed on May 12, 2005. This application is also acontinuation-in-part application of U.S. patent application Ser. No.10/997,845, entitled “Object Detection Method and Apparatus”, filed onNov. 24, 2004, which claims the benefit of the filing of U.S.Provisional Patent Application Ser. No. 60/525,637, entitled “ObjectDetection Method and Apparatus Employing Polarized Radiation andArtificial Intelligence Processing”, filed on Nov. 25, 2003, and whichis also a continuation-in-part application of U.S. patent applicationSer. No. 10/340,016, entitled “Signal Processing for Object DetectionSystem”, filed on Jan. 9, 2003, which is a continuation-in-part of U.S.patent application Ser. No. 10/060,641, entitled “Signal Processing forObject Detection System”, filed on Jan. 29, 2002, and issuing on Nov.30, 2004 as U.S. Pat. No. 6,825,456, which is a continuation-in-part ofU.S. patent application Ser. No. 09/318,196, entitled “Object DetectionSystem”, filed on May 25, 1999 and issued on Jan. 29, 2002 as U.S. Pat.No. 6,342,696. The specifications of all said applications areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention is a method and apparatus for remotely detectingthe presence of an object, including but not limited to a concealedweapon such as a gun or bomb. The invention further comprises novelsignal processing methods and apparatuses for providing high reliabilityobject detection.

BACKGROUND OF THE INVENTION

Note that the following discussion refers to a number publications andreferences. Discussion of such publications herein is given for morecomplete background of the scientific principles and is not to beconstrued as an admission that such publications are prior art forpatentability determination purposes.

On 20 Apr. 1999, two students at the Columbine High School in Littleton,Colo. opened fire on their classmates and teachers with assault weapons.Twelve teenagers and one teacher were killed, and dozens of others werewounded. Tragic acts of violence like the Littleton massacre occur alltoo often in present day America. The Federal Bureau of Investigationreports that every year, criminals in the United States use firearms tocommit over 2.4 million robberies, 5.6 million assaults, and 165,000rapes. (See American Firearms Industry Journal, published by theNational Association of Federally Licensed Firearms Dealers) The Centerfor Disease Control has collected data showing that 247,979 “firearmdeaths” were recorded in the United States during the years 1986-1992.(Data compiled by the Center to Prevent Handgun Violence.) Furthermore,in recent years, a new threat has evolved; that is the suicide bomber.These are more dangerous and more devastating and because of the natureof their weapon, it is imperative that they be detected at a longdistance.

Many previous efforts to reduce the threat posed by the criminal use offirearms have met with limited success. In the past two decades, veryexpensive x-ray equipment has been installed in major airports. Themachines are generally capable of detecting a metal gun in a veryspecialized, closed environment. This type of equipment requires a fixedinstallation, occupies a very large space, is close-range and may costhundreds of thousands or even millions of dollars.

None of the complex concealed weapon detectors that are currentlyavailable in the commercial market are compact, lightweight, portable,easy to use, long-range and highly reliable. The development of such adevice would constitute a revolutionary achievement and would satisfy along felt need in the fields of law enforcement and security.

Earlier versions of the present invention are described in U.S. Pat. No.6,243,036, issued Jun. 5, 2001, entitled “Signal Processing for ObjectDetection System”, U.S. Pat. No. 6,359,582, issued Mar. 19, 2002,entitled “Concealed Weapons Detection System”, International PatentApplication Number PCT/US97/16944, entitled “Concealed Weapons DetectionSystem”, published on Mar. 26, 1998 as International Publication NumberWO 98/12573, and International Patent Application Number PCT/US00/14509,entitled Signal Processing for Object Detection System”, published onDec. 14, 2000 as International Publication Number WO 00/75892. Thespecifications and claims of these references are incorporated herein byreference.

SUMMARY OF THE INVENTION

The present invention is a method of determining the presence of anobject associated with a target, the method comprising the steps of:illuminating the target with polarized illuminating radiation;collecting first radiation reflected from the target which has a samepolarization as the illuminating radiation; collecting second radiationreflected from the target which has an opposite polarization from theilluminating radiation; and employing a weighted plurality of criteriaof the first radiation and the second radiation to determine thepresence of the object. The employing step preferably comprisesemploying a weighted plurality of criteria of the collected radiationconverted to a time domain by a Chirp-Z Transform process. The employingstep preferably comprises employing a magnitude spread of one or both ofthe first radiation and the second radiation at a plurality of times.The employing step further preferably comprises employing a plurality ofcriteria selected from the group consisting of a first magnitude of thefirst radiation at zero time after conversion to the time domain by theChirp-Z Transform process, a second magnitude of the second radiation atzero time after conversion to the time domain by the Chirp-Z Transformprocess, and a difference between the first magnitude and the secondmagnitude. The employing step optionally comprises employing a time ofarrival difference between the first radiation and the second radiationand or a measurement of the shape of a curve, preferably the ratio ofthe peak value of the curve to the total area under the curve, of one orboth of the first radiation and the second radiation in the time domainor frequency domain.

The method is preferably repeated a plurality of times, and furthercomprises the step of combining results of each performance of themethod. The method preferably further comprises the step of training aneural network on calibration data, and the employing step preferablyfurther comprises the step of using the neural network to autonomouslydetermine presence of the object.

The target preferably comprises a person and the object preferablycomprises a concealed weapon, preferably selected from the groupconsisting of a knife, firearm, gun, bomb, explosive device, and suicidevest.

The present invention is also an apparatus for detecting an objectassociated with a target, the apparatus comprising a transmit antennafor illuminating the target with polarized illuminating radiation, afirst receive antenna for collecting first radiation reflected from thetarget which has a same polarization as the illuminating radiation, asecond receive antenna for collecting second radiation reflected fromthe target which has an opposite polarization from the illuminatingradiation, and a processor for employing a weighted plurality ofcriteria of the first radiation and the second radiation to determine apresence of the object. The processor preferably employs a weightedplurality of criteria of the collected radiation converted to the timedomain by a Chirp-Z Transform process, and preferably employs amagnitude spread of one or both of the first radiation and the secondradiation at a plurality of times. The processor also preferably employsa plurality of criteria selected from the group consisting of a firstmagnitude of the first radiation at zero time after conversion to thetime domain by the Chirp-Z Transform process, a second magnitude of thesecond radiation at zero time after conversion to the time domain by theChirp-Z Transform process, and a difference between the first magnitudeand the second magnitude.

The processor further preferably employs a time of arrival differencebetween the first radiation and the second radiation, preferably employsa shape of a curve of one or both of the first radiation and the secondradiation in the time domain or frequency domain, and preferably employsa variation in time of one or both of the first radiation and the secondradiation. The processor preferably combines results from a plurality ofapplications of the illuminating radiation to the target. A singledual-polarized antenna optionally comprises said first receive antennaand said second receive antenna.

The target is preferably a person. The object is preferably a concealedweapon, preferably selected from the group consisting of a knife,firearm, gun, bomb, explosive device, and suicide vest. The processorpreferably employs a neural network to automatically detect the presenceof the object, preferably assigning a value to each of the criteria anddetermining the presence of the object based on a combination of valuesof the criteria.

The invention is also a method for detecting an object concealed on atarget, the method comprising the steps of transmitting continuous waveelectromagnetic radiation to the target at selected frequencies within afrequency range, receiving two orthogonally polarized signals reflectedfrom the target at each selected frequency, measuring the amplitude andphase of the reflected signals, generating a frequency domain waveform,transforming the waveform to a time domain in a time windowcorresponding to a distance to the target, and processing the timedomain waveform to determine whether an object is concealed on thetarget. The method preferably further comprises the step of filteringthe frequency domain waveform, preferably using a Hamming filter. Thetransforming step preferably comprises using a Chirp-Z transform. Thetransforming step preferably results in range gating of the signal.

The method preferably employs a plurality of units, each unit performingthe transmitting and receiving steps. The units preferably perform thetransmitting step sequentially. Continuous wave transmission of eachunit is preferably interrupted to allow the other units to transmitradiation, in which case the time of continuous wave transmission ofeach unit is long compared to the transit time of the radiation from theunit to the target and back to the unit. The units preferably performthe receiving step sequentially, wherein each unit preferably receivessignals transmitted solely by its own transmitter. Each unit optionallyreceives signals transmitted by a transmitter of another unit. Reflectedsignals received by the units are preferably compared. For example, thesignal amplitudes received by each unit may be averaged. The units arepreferably disposed at different heights relative to the target, and arepreferably oriented to illuminate the target from different viewpoints.The object is preferably selected from the group consisting of weapon,firearm, bomb, suicide vest, explosive, merchandise, tag, work inprocess, inventory, and manufacturing product. The method is optionallyperformed to prevent shoplifting or inventory theft.

An object of the present invention is to provide a detection devicewhich is preferably compact, lightweight, long-range, portable andbattery-operable. This enables a preferred embodiment of the device tobe hand-carried unit that could be used by law enforcement officersand/or military or security personnel, for example to determine if aparticular individual is armed.

An advantage of the present invention is that the power levels radiatedby the present invention are preferably much lower than conventionalradar systems or those generated by x-ray or other imaging systems thatare currently employed to detect objects at the entry of an airport or acourtroom. For the present invention the average power density at thetarget is orders of magnitude below the safety limit for non-ionizingradiation.

Other objects, advantages and novel features, and further scope ofapplicability of the present invention will be set forth in part in thedetailed description to follow, taken in conjunction with theaccompanying drawings, and in part will become apparent to those skilledin the art upon examination of the following, or may be learned bypractice of the invention. The objects and advantages of the inventionmay be realized and attained by means of the instrumentalities andcombinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the specification, illustrate several embodiments of the presentinvention and, together with the description, serve to explain theprinciples of the invention. The drawings are only for the purpose ofillustrating a preferred embodiment of the invention and are not to beconstrued as limiting the invention. In the drawings:

FIG. 1 a illustrates a simple wave;

FIG. 1 b illustrates a simple wave that is vertically polarized;

FIG. 1 c illustrates a simple wave that is horizontally polarized;

FIG. 2 provides a block diagram of one embodiment of a transmission anddetection circuit;

FIG. 3 portrays persons carrying a gun in different locations on thebody;

FIG. 4 a is a graph showing the radar cross section of a handgun,plotting reflected energy in dBsm versus frequency;

FIG. 4 b is a graph showing the radar cross section of a human body,plotting reflected energy in dBsm versus frequency;

FIGS. 5 and 6 are graphs which supply information concerning thereflectivity of the human body when illuminated with radio waves in the2.59 to 3.95 GHz and 7.0 to 10.66 frequency bands;

FIG. 7 is a pictorial representation of a preferred embodiment of themethod of the present invention. The two graphs at the right of thedrawing show that an object such as a weapon may be detected bycomparing the time domain difference in amplitudes of two sets ofwaveforms which correspond to reflected radio waves having differentpolarizations. In both the upper and the lower graphs, the two waveformsrepresent the vertically and horizontally polarized radio wavesreflected back to the detector;

FIGS. 8 and 9 are actual test equipment plots of two pairs of timedomain waveforms generated during a handgun detection experiment. InFIG. 8, the person was not carrying a gun; in FIG. 9, the same personwas carrying a handgun, and the distance between the maxima of the twocurves is much closer;

FIG. 10 is a general illustration of the phase and amplitude responseused for the Complex Chirp-Z Transforms that are employed in a preferredembodiment of the present invention;

FIG. 11 depicts an operational system flow diagram of a preferredembodiment of the present invention; and

FIG. 12 is a circuit diagram of an interrupted CW embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Best Modes for Carrying Out theInvention

The present invention comprises methods and apparatus for detecting thepresence of an object at a distance. One embodiment of the invention maybe used to locate a concealed firearm and/or bomb carried by a person.The invention may be used to help keep weapons out of any secure area or“Safe Zone™,” such as a school, bank, airport, embassy, prison,courtroom, office building, retail store or residence. The term “SafeZone™” is a Trade and Service Mark owned by the Assignee of the presentPatent Application, The MacAleese Companies, doing business as SafeZone™ Systems, Incorporated.

The object is preferably associated with a target, for example a personapproaching a doorway, and is detected preferably using polarized lowpower radio waves. As used throughout the specification and claims, theterm “target” means something toward which illuminating radiation ispointed, including but not limited to a person, backpack, luggage, bag,shrub, and the like. As used throughout the specification and claims,the term “object” means a physical item that is carried on, worn,concealed on, physically attached to, or coupled or otherwise associatedwith a target, including but not limited to a weapon, knife, firearm,gun, pistol, rifle, bomb, suicide vest, shrapnel, wiring, and the like.

As radio waves travel through the air, they travel in a way similar towaves of water moving across the surface of the ocean. The shape of asimple radio signal can be depicted as a repeated up and down movementor vibration, as shown in FIG. 1 a. This up and down motion of the wavetakes place in three dimensions. The simple wave (W) propagates. A wavewhich is polarized parallel to the plane of propagation is called ahorizontally polarized wave. A wave which is polarized perpendicular tothe plane of propagation is called a vertically polarized wave. Theheight or intensity of the wave W is called the amplitude (A) of thewave.

FIG. 1 b exhibits a wave which is vertically polarized, while FIG. 1 cdepicts a wave which is horizontally polarized. Vertical and horizontalpolarizations are said to be orthogonal forms of polarization. Otherterms that may be used to describe the relationship between waves thatare vertically and horizontally polarized are perpendicular, opposite,cross-polarized, or main and complementary. The term primarily used inthis document to denote orthogonal polarizations is cross-polarized, orcross-pol or X-pol for short. The idea of polarization is applicable toall forms of transverse electromagnetic waves, whether they are radiowaves at microwave frequencies, or light waves such as those emitted bya flashlight.

The power levels radiated by the present invention are much lower thanconventional radar systems or than those generated by x-ray or otherimaging systems that are currently employed to detect objects at theentry of an airport or a courtroom. In fact, the average power densityat the target for the preferred embodiment of this invention is ordersof magnitude below the safety limit for non-ionizing radiation.

The present invention preferably operates in the GHz frequency bands.Different radio frequencies offer different benefits and disadvantagesfor object detection. In the United States, operating frequencies ofradio devices are regulated by the Federal Communications Commission.Each country across the globe has similar regulatory bodies thatallocate and administer the use of the radio spectrum. Although thespecification includes specific references to particular frequencyranges, the system may be beneficially implemented using a wide varietyof electromagnetic radiation bands and is not limited by thespecifically disclosed ranges.

FIG. 2 presents a non-limiting example of a schematic block diagram ofcircuitry for implementing a preferred embodiment of the invention. Lowpower radio transmitter 12 is coupled via first directional coupler 13to modulator 14, filter 16, and transmitter output amplifier 18, whichis connected to transmit/receive antenna 80 through transmit/receiveswitch 20 and pre-selector 22. Transmit/receive switch 20 is alsosynchronized with range gate switch 90 through controller 26.Transmit/receive antenna 80 and receive antenna 82, which detects energyat an orthogonal polarization, collect energy reflected back from thetarget. Alternatively, a single dual-polarized antenna may optionally beused. Polarity selection switch 24 in the receive path selects eitherthe horizontally or vertically polarized antenna or port.Transmit/receive antenna 80 preferably both transmits the signal inhorizontal polarization and receives the reflected horizontal or co-polsignal, and receive antenna 82 preferably receives the vertical or X-polreflected signal in vertical polarization. Polarity switch 24 determineswhich signal is fed to the receiver at any given time. Pre-selectorfilters 22, 23 are bandpass filters to prevent out of band signals fromentering the receiver and possibly causing spurious responses orsaturating the amplifiers and thereby preventing normal operation.Pre-selector filter 22 in the co-pol path also attenuates undesirableharmonics of the transmitter from being transmitted.

Controller or processor 26, preferably comprising start/stopislopeprogramming, is used to control transmitter 12 in conjunction with localoscillator 30. The output of pulse waveform generator 28 is connected tomodulator 14. The output of local oscillator 30 is fed to mixer 32 viasecond directional coupler 11. An output of transmit/receive switch 20is also fed to mixer 32 through polarity selection switch 24, filter 36and receive low-noise amplifier 34. A preferably digital output fromprocessor 26 is conveyed to intermediate frequency gain controlamplifier 40, which also receives the main signal input from a mixer 32through band pass filter 41. The output from amplifier 40 then passesthrough range gate switch 90, high pass filter 42 and to power divider44. Range gate control 21 and range gate switch 90 provide time gatingso that only a signal from a reflector (i.e. target or object) that isat the desired distance from the apparatus is processed. Signals fromother objects that arrive at different times are ignored. Power divider44 splits the signal into two outputs. One output is amplitudedemodulated in detector 46, producing narrow pulses which are passedthrough filter 48, video amplifier 50, gated sample and hold stretcher52, and then digitized in analog-to-digital converter 54 before beingfed back to processor 26. The second output from power divider 44 is fedvia power splitter 64 to phase detectors 65, 66 so that the phase shiftof the returned signal can be measured at the same time as theamplitude.

Since phase information is very important in order to perform complexfrequency to time domain transforms, the phase of the reflected signalsis preferably measured. As phase is a relative term, this isaccomplished by first establishing a reference signal by mixing samplesof the transmit and local oscillator signals. A sample of the transmitsignal is taken from transmitter 12 through directional coupler 13 andfed to mixer 9 along with a sample of the local oscillator signal takenfrom local oscillator 30 through directional coupler 11. The output ofmixer 9 is band pass filtered by filter 8 and then limited by limiteramplifier 7 to stabilize its amplitude over the tuning range. Thelimited signal is fed to quadrature hybrid 72 which outputs two signalsthat are equal in amplitude but phase shifted relative to each other by90°. One of the outputs is fed to first phase detector 65 and the otheroutput is fed to second phase detector 66. Two offset phase detectorsare used to unambiguously cover a range of 360 degrees. The outputs ofphase detectors 65, 66 are analog and subsequently digitized inanalog-to-digital converters 68 and 70. The digitized signals are fed tocontroller 26 for subsequent processing.

The unit of measurement “dBsm” is used to quantify reflected radiationand is based on a unit of measurement called the decibel, abbreviated“dB.” Decibels are used to compare two levels of radiated or reflectedpower. As an example, if a person listening to a radio is very close tothe antenna tower of a radio station, the power level would be veryhigh. If the same person were many miles away from the same antennatower, the strength of the received radio waves would be much lowerbecause of the increased distance. Decibels could be used to quantifythis ratio of power levels as a single number. Unlike common fractions,which are simply one number divided by another number, decibels are alogarithmic form of measurement, which is highly useful since they areused to compare very large differences in numbers. Since radiated powerlevels can vary over such large ranges, a logarithmic scale is usedinstead of a more common linear scale. The difference in two powerlevels in decibels is calculated as follows:dB=10 log(P _(X) /P _(Y))  (1)where P_(X) is a first power level, and P_(Y) is a second power level.When the two received radio signals are compared using decibels, thereduction in the power of the signal that is received at the greaterdistance is said to be a certain number of decibels lower than the powerlevel at the closer location.

The “radar cross section”, or RCS, is a measure of the size of anobject. When radio waves are generated and then directed toward anobject, some portion of those transmitted waves pass through the object,another portion of those waves are absorbed by the target, and a thirdportion of the transmitted waves are reflected back toward thetransmitter. The larger the portion of reflected waves, the greater isthe radar cross section of an object. An object that has a relativelylarge radar cross section is therefore relatively easier to detect,compared to an object that has a smaller radar cross section. Themagnitude of the measured radar cross section of an object dependslargely on its reflectivity, and on the spatial orientation of theobject. For example, suppose a radar station on the shoreline is lookingfor ships at sea nearby. A ship which is traveling parallel to thecoastline is easier to detect than a similar vessel that is sailing awayfrom land, since the radar waves that hit the first ship broadsidebounce back to the radar station with greater intensity than those whichreflect off of the smaller stern of the second ship. Thus the firstship, which is oriented “sideways” to the direction of travel of theradar waves, has a larger radar cross section than the second ship,whose stern presents a smaller target to the radar waves.

When the present invention is used to detect an object like a handgun,the detection is more easily accomplished when the handgun is orientedin a way that presents a relatively larger radar cross section to thedetector. For example, a gun that is tucked behind a person's beltbuckle so that the side of the gun is flat against the waist presents alarger radar cross section than a weapon holstered on the hip with thegun barrel pointing toward the ground and the grip pointing forward orback. FIG. 3 is a pictorial rendition of two persons carrying handguns.On the left side of the figure, a person is shown with a gun held inplace either in front or in back of a belt. On the right side of thefigure, another person is shown with a gun carried in a bag, pouch, orholster situated on the hip at the person's side. Note that in order forthe radar cross sections of the guns in both positions to be similar,the figure must be turned, or facing a different direction, relative tothe detector.

The radar cross section when compared to one (1) square meter isexpressed in a decibel unit of measurement, “dBsm”, as follows:RCS(in dBsm)=10 log (AG)=10 log A+10 log G  (2)where A is the area of the target in square meters and G is the gain ofthe target on reflection. This expression assumes that the area is flatrelative to the wavelength of operation, and that the area is uniformlyilluminated by radio waves. If the side of a square area is “a” inmeters, then the area becomes “a²” in square meters. For a surface whichis flat relative to the wavelength of operation,G=4πa ²/λ²  (3)where the wavelength λ is equal to 0.3/f meters and f is frequency inGHz. ThusRCS(in dBsm)=10 log (4πa ⁴ f ²/0.09)  (4)

This expression indicates that if the size “a” of the side is doubled,the reflection increases by 12 dBsm, or in linear power units, the crosssection is 16 times greater. If the frequency doubles, the reflectionbecomes 6 dBsm greater or 4 times as great in linear power units. Notethat RCS in dBsm increases as 20 log(f). Complicated edge effects areignored in this description. For example, the radar cross section of a6″×6″ plate at 1 GHz is −11.3 dBsm. Since the factor G, or gain,increases proportional to f², an increase from 1 to 10 GHz increases thevalue to 8.7 dBsm, a difference of 20 dB. However, typical weaponsshapes are significantly non-planar relative to the radar wavelength, sovery little increase is actually realized.

The data in Table 1 is the radar cross section of a metal .357 caliberhandgun illuminated by electromagnetic radio waves in several frequencybands. These data were established to calibrate the detector equipmentand to provide reference measurements. The test configuration was: oneport RCS measurement, 16 averages, time domain gating, and reduced IFbandwidth.

TABLE 1 Frequency Band (MHz) Radar Cross Section  500-1000 −15 dBsm1000-1750 2650-3000 −10 dBsm 2890-3250  9500-10660

Similarly, FIG. 4 a provides data on the radar cross section (RCS) of a0.357 caliber pistol for transmitted radiation spanning the 2650 to 3000MHz frequency range. The curve shows that for a gun oriented in thebroadside position, meaning that the longest dimension of the gunextends sideways in the plane of the transmitted radio wave, the RCSvaries from about −8 dBsm to −11 dBsm over this frequency range. FIG. 4b represents a body return, or the RCS of a human body without a weapon,in the same frequency band as FIG. 4 a. The average radar cross sectionacross the band is −3 dBsm or approximately 8 dB stronger than theaverage gun return of −11 dB.

FIGS. 5 and 6 provide measurements of the reflection of radio waves of aperson in the test chamber. FIG. 5 contains empirical data thatindicates that when a person is illuminated with radiation, about 63% ofthe radio wave energy is reflected back from the body between 2.59 to3.95 GHz. FIG. 6 shows that about 32% is reflected back between 7.0 to10.66 GHz.

In general, the present invention preferably relies on the physicalphenomenon of reflection in which a horizontally polarized incident beamwill be partially reflected back as vertical polarization. Thepercentage of energy converted to vertical polarization depends on theshape of the object in the plane normal to the direction of incidence.If the object has a cross sectional shape that has both vertical andhorizontal components, then a vertically polarized component will berealized even though the object is irradiated by horizontally polarizedwaves. This vertically polarized component is referred to herein as the“cross-pol”, while the horizontally polarized reflection is referred toas the “co-pol”. These terms are reversed if the target and object areirradiated with a vertically polarized incident beam.

As noted above, the difference in backscatter between a .357 hand gunand the human body is approximately −8 dB on the average. In arithmeticterms this means that the combined gun plus body signal will increaseonly 1.4 dB over the case without a gun. Given that the human bodyvariance is on the order of 6 dB, it is not hard to understand why a gunwould be difficult to detect. The major bones in the human body arevertical so it is not surprisingly then that the cross section is higherfor incident vertical polarization. This is also true for the vastmajority of zippers in clothing.

If incident horizontal polarization is used, the body cross sectionreduces by approximately 6 dB, and the now vertically polarized crosspolarization reduces a like amount. However, the cross polarization of aweapon stays relatively constant. This means that the 1.4 dB differencecan now become 7.4 dB, on the average, thereby reducing the effect ofthe variation from one body to another. Thus, when the target is ahuman, it is preferable to transmit horizontal polarization, and toreceive both horizontal and vertical polarization.

FIG. 7 depicts typical operation of a preferred embodiment of thepresent invention. Persons entering a protected space, or “Safe Zone™”,are illuminated with radio waves which are in this instance horizontallypolarized. A portion of these radio waves are absorbed, while some arereflected back toward the transmitter. When the transmitter illuminatesa person without a gun, the two curves in the upper graph in FIG. 7result. These two curves represent the amplitude of the horizontallypolarized energy reflected back to the detector (the upper curve labeled“α”) and the amplitude of the vertically polarized energy reflected backto the detector (the lower curve labeled “β”) in the time domain afterapplying a Chirp-Z transform (as described below).

The lower graph shown in FIG. 7 contains two curves produced when aperson is carrying a handgun that is sensed by the detector in the timedomain. As in the upper graph, the two curves represent the energy levelof horizontally polarized radio waves reflected from the person (theupper curve labeled “γ”) and the energy level of vertically polarizedradio waves reflected back from the person (the lower curve labeled “δ”)in the time domain. The gap between the maximum amplitude of the curves,labeled “Delta B,” is usually somewhat narrower than the gap in theupper graph, labeled “Delta A”. In general, when the person has a gun,or any other object that presents a substantial reflective presence, thecomponent of vertically polarized energy that is reflected back from theobject increases.

FIGS. 8 and 9 are measured time domain test equipment plots of two pairsof waveforms generated during a handgun detection experiment. In FIG. 8,the person was not carrying a gun, and the maximum values of the twocurves are 29.6 dB apart. The incident polarization is horizontal andthus the receive polarization for horizontal is greater than the receivepolarization for vertical. In FIG. 9, the same person was carrying ahandgun, and the distance between the maximum values of the two curvesis now only 7.9 dB, indicating the presence of a gun.

While the decrease in the difference in amplitudes between the tworeceived polarizations was quite dramatic for this one test, in othercases it may be quite small; thus this measurement cannot always berelied upon. Therefore, additional parameters or criteria must beconsidered when making a decision as to the presence of a weapon.Furthermore, in the real world (which is not an anechoic chamber)signals fade due to multipath effects caused by ground reflections andclutter in the surrounding environment. Multipath effects are preferablyminimized by having the radar sweep over a wide range of frequencies, asdescribed above, since a cancellation at one frequency will not have thesame effect at another. Sweeping over a wide range of frequencies offersan additional advantage in that the wider the spectrum used in thefrequency domain, the better the resolution for time and amplitude inthe time domain after the appropriate transform is applied.

Another criterion which is preferably used in the present invention isthe relative timing of the peaks of the two return signals changes. Whena person has a gun or a bomb, a good portion of the vertically polarizedsignal, which is mostly created by the weapon, moves forward in timerelative to the horizontal return, which is mainly reflected from thebody. Such a time shift is another parameter which contributes to theprobability of the detection of a weapon.

Furthermore, the shape of both of the polarized returns tends to spreadout more when a person is armed because part of the reflection comesfrom the weapon and part from the body. Therefore, measuring the ratioof the peak value to the area under the curve for each return preferablycontributes to determining the probability of object detection.

Finally, the absolute amplitude of the co-pol return (typicallyhorizontally polarized) tends to be greater when a person is armed dueto the reflection from the weapon but this, in itself, is not a heavilyweighted parameter because of the variation in the sizes of differentpeople. If the co-pol return were significantly greater than the valuethat is normal for a very large person, then this alone would indicatean abnormality with a particular individual, possibly indicating aconcealed object. This parameter is of great significance when a personis wearing a bomb comprising shrapnel.

The present invention is preferably implemented by solving an algorithmwhich uses a standard set of stored values that represent the signalswhich are reflected from persons who are not carrying concealed weapons.This data, which is preferably measured and compiled using a number ofpersons, furnishes the information represented in the upper graph shownin FIG. 7, and in FIG. 8. A standard set of stored values that representthe signals which are reflected from persons who are carrying concealedweapons is also used. This data, which is also preferably measured andcompiled using a number of persons, furnishes the informationrepresented in the lower graph shown in FIG. 7, and in FIG. 9. In anadvanced implementation of the invention, the detector is capable ofadapting to its environment by progressively and continuously learningabout the reflected signals that are produced by many persons enteringthe “Safe Zone™” who are not carrying weapons. This can be accomplishedby utilizing one of any number of learning systems, including but notlimited to a neural network.

One of the most difficult issues in the gun detection scheme of thepresent invention is the variance of the human body. All data shown todate used amplitude input only to convert from the measured frequencydomain to the displayed time domain plots. However, as discussed abovethis is inadequate to provide a reliable indication of the presence of aweapon or other object.

U.S. Pat. No. 6,342,696, entitled “Object Detection System”, disclosesnovel methods and apparatuses for detecting concealed weapons, includingutilization of a time domain method in which the amplitude differencebetween the co-polarized and cross-polarized returns from a target areais used to determine if a weapon is present. An algorithm employing aComplex Chirp-Z Transform (CZT), which can accommodate both amplitudeand phase data by incorporating phase information into thetransformation, is preferably employed to improve the sensitivity ofdetecting objects. A CZT is a mathematical expression that is used toconvert information about frequency to information about time, i.e. toconvert from the frequency domain to the time domain. The CZT is ageneralization of the Z transform, which is a discrete form of theLaplace Transform.

Measuring the phase of the polarized waves reflected from a person whomay be carrying a concealed weapon is important because the polarizedwaves reflected from a concealed weapon and the polarized wavesreflected from a human body behave quite differently. In general, thereflections from a concealed weapon, while not constant, vary within arelatively confined range. In contrast, the reflections from a humanbody vary in time because the body has depth and the reflections aregenerated at various depths in the body; the reflections are thereforenon-planar. The centroid of the transformed return is at a point that isbelow the surface of the body. The present invention preferably exploitsthis characteristic by using signal processing methods to distinguishthe relatively compacted signals from a concealed weapon from thegenerally time/distance varying signals from a human body. The result ofusing the non-planar data is a reduction in the return from the humanbody, increasing system sensitivity and the ability of the invention todetect concealed weapons. Thus the CZT helps to separate a first signalwhich is generated by radiation reflected from an object from a secondsignal generated by radiation reflected from a target such as a humanbody.

A complex transform requires the knowledge of the relative phase shiftof each frequency component; thus, in order to use a CZT, both amplitudeand phase information must be collected during the measurement period.Therefore, a phase detector has been introduced into theinstrumentation; see FIG. 2. The phase detector is actually built in twosections, each being fed with identical signals that are offset 90° fromeach other. Such a quadrature detector is required to eliminate theambiguities in the phase detector as a single section unit repeats thevalues at different quadrants of the circle. Since only one frequencyexists at a given time, it was preferred to measure the phase relativeto the transmitter signal. The phase measurement, preferably of thecross-pol returned signal, is preferably performed at the IF signal. Asan alternative, the phase measurement can be performed at the radiofrequency (RF) signal without any significant difference; however, it ismore difficult and expensive to obtain accurate measurements at RF. Ineither method, a phase discriminator is used to measure the phase of thereturned signal relative to the transmitted signal. However, a concernexisted as to maintaining coherency at IF.

Such coherency can be maintained preferably by employing an additionalchannel to provide a reference at the precise IF of the return signal.This is accomplished preferably by sampling both the transmit and localoscillators and mixing them to produce the phase detector reference.Noise is minimized because the delay in receiving the return is onlynanoseconds due to the close proximity of the target. A single phasedetection channel is preferably used and is time multiplexed to permitseparate phase measurements of the co- and cross-polarity channels. Analternative method to create a stable reference is to employ a stableoscillator operating at the IF and synthesize the local oscillator usingthe IF reference and transmit oscillators.

Generalized depictions of sample cross-pole amplitude and phaseresponses from a human body are presented in FIG. 10. This informationis processed using a Complex Chirp-Z Transform. The waveforms in FIG. 10can be defined as follows:S(f)=A _(fil(f)) ×A _((f)) e ^((2πft+δ(f)))where:

A_(fil(f))=amplitude response of the bandpass filter in the frequencydomain;

A_((f))=amplitude response of the cross-pole return in the frequencydomain;

f=frequency in gigahertz;

t=time in nanoseconds; and

P_((f))=δ(f)=phase response of the cross-pole return in the frequencydomain.

The frequency band of interest is broken into segments or bins. Thenumber of bins “N” can be any practical value, from zero to a numberapproaching infinity.

The data acquired from the radar signal consists of the magnitude andphase of the reflected signal at each frequency. In order to be useful,these values must be converted into values representing magnitude vstime. The following are definitions of radar parameters:

N=number of frequency samples at which measurements are made;

F_step=size of frequency steps between samples; and

F_span=total frequency span (N×F_step).

The standard method of converting a frequency signal to time is throughthe use of an Inverse Discrete Fourier Transform (IDFT). An IDFTtransforms N frequency samples into N time samples. The resulting timesamples are evenly spaced from time 0 to time=1/F_step with a resolutionof 1/F_span. The IDFT is defined as:

${x(n)} = {\sum\limits_{k = 0}^{N - 1}\;{{X(k)}{\mathbb{e}}^{\;\frac{j\; 2\pi\;{nk}}{N}}}}$Where X(k) are the N frequency samples (complex) and x(n) are the N timesamples. For example, with N=128 and F_step=7.8125 MHz, the IDFT givesus 128 time samples each spaced 1 nanosecond apart from time 0 to 128ns. This method, however, proves to be inadequate for two reasons.First, we are not interested in all of the time from 0 to 128 ns, butonly on the very small (˜10 ns) time slice where the reflections fromthe target and object are present. Second, the 1 ns time resolution istoo coarse to make the precision time measurements which are preferablefor the present invention.

These two inadequacies are resolved by using the Chirp-Z Transform toconvert from frequency to time. The Chirp-Z operates on the sameprinciple as the IDFT, but permits the ability to zoom in on a region ofinterest. The forward (time to frequency) Chirp-Z Transform is given by:

${X(k)} = {\sum\limits_{n = 0}^{N - 1}\;{{x(n)}A^{- n}W^{nk}}}$where

-   -   A=A₀e^(j2πθ) ⁰    -   W=W₀e^(j2πφ) ⁰        and    -   k=0, 1, . . . , M−1

A₀ determines the Chirp-Z initial radius;

W₀ determines the “spiral factor” of the Chirp-Z transform;

θ₀ determines the starting position as a fraction of the whole interval;

φ₀ determines the step size as a fraction of a whole interval;

N is the number of input (time) values; and

M is the number of output (frequency) values.

By using the above equations and choosing appropriate values for A₀, W₀,θ₀, φ₀, N and M the interval and resolution of our transform can bechosen. For the present invention, A₀ and W₀ are preferably set to 1.

The above equations are for the forward (time to frequency) Chirp-ZTransform. The inverse Chirp-Z Transform is computed by taking thecomplex conjugate of the transform of the complex conjugate of thefrequency data. For example, to calculate the values in time from 30 nsto 40 ns with N=128 and M=64 we would set:

θ₀=30/128 (start time divided by total time) and

φ₀=(10/128)/64 (time sweep divided by total time divided by number ofoutput samples)

The data processing for the CWD system preferably comprises thefollowing steps:

1. Acquire the frequency magnitude and phase values;

2. Apply a Hamming window to the values;

3. Convert the magnitude and phase values into real and imaginaryvalues;

4. Conjugate the complex values;

5. Perform the Chirp-Z transform;

6. Conjugate the result; and

7. Convert the result from real and imaginary values into magnitudevalues.

The following references provide further details on the Chirp-Ztransform: “The Chirp-Z Transform Algorithm and Its Applications”, L.Rabiner, et al, MIT Lincoln Laboratory, Bell System Journal, May-June1969; “Using the Inverse Chirp-Z Transform for Time Domain Analysis ofSimulated Radar Signals”, Dean A. Frickey, Idaho National EngineeringLaboratory; “Linear Filtering Approach to the Computation of DiscreteFourier Transform”, L. Bluestein, GT&E, IEEE Transactions on Audio andElectroacoustics, December 1970; and Frederic deCoulon, Signal Theoryand Processing. The entirety of these references is incorporated hereinby reference.

The main advantage of the Chirp-Z Transform over the Fast FourierTransform is that very accurate time shift data is available with aresolution of tens of picoseconds. This provides information on thespatial position of the generation of the cross-pol return relative tothe co-pol return with a resolution of less than 1 inch. This in turnprovides information as to whether the cross-pol was generated withinthe body of a human or by an object in front of, or otherwise on thesurface of, the body. The neural network described below preferablyutilizes this information as part of the decision making process.

In order to separate the two radar returns from the target, which are ofdifferent polarities, a preferred embodiment of the present inventionemploys an antenna that has dual feeds, one for the co-pole and theother for the cross-pole, using two separate antennas. The first antennatransmits preferably horizontally polarized waves and receives in thesame polarity (co-pol). The second antenna preferably only receives inthe opposite polarity (cross-pol) and does not transmit. Normally suchradars use two receiver channels to keep the two received signalsseparated. Alternatively the present apparatus may multiplex the signalsand use a microwave switch to alternately connect the receiver channelto each of the antennas, thereby saving the cost of a second receiver.This savings is quite substantial. This approach can be applied as wellto an antenna design that uses a separate feed for each polarity.

Time multiplexing is preferably accomplished by adding a SP2T switch atthe input to the receiver where each input is fed by each antenna. Thetransmitted signal is preferably a burst of pulses separated by aperiod, such as about 1 microsecond, as opposed to a single pulse. Thereturns from each pulse in the group are averaged to negate anyunexplained occasional strange readings. A group of preferably 3 to 5pulses is adequate for this application.

Due to the speed of the measurements relative to the time that it takesa human body to move, the readings can be taken in a variety ofsequences as long as a set is completed in less than an arbitrary timeof preferably one millisecond. This allows the system to be designed inthe most simplified form, as it will not matter if the co- and cross-polmeasurements are made at each frequency, or if all the co-polmeasurements are made first on one frequency sweep and then thecross-pol measurements on an alternate frequency sweep. The lattermethod allows the use of a polarity switch to select the co-pol returnand then the cross-pol return and use only one receiver to measure both.A relatively slow switch, with a switching time of 50 to 100nanoseconds, can then be used. A repetition rate of preferablyapproximately 10 KHz would allow a measurement sweep to be completed ina reasonable time.

A weighting function is preferably applied to the various types of datacollected. In addition to the difference between the cross-polaritymagnitude at zero time after being converted to the time domain and theco-polarity signal magnitude at zero time after being converted to thetime domain there are other pieces of data that are also of value. Forexample, the values of both the magnitude and phase of the co- andcross-polarity returns provide some indication as to the amount of metal(or other radar reflective material) on a person, even though a largerperson produces a return that is about 3 dB greater than a smallerperson. However, a person with a bomb may produce a much greater returnthan a large person. In addition, a plurality of frequency sweeps foreach reading, in the course of approximately 300 milliseconds isoptionally taken, and an average is preferably calculated. A safesubject (no weapon of any kind) typically produces a significantvariation in the five values (large standard deviation), while a personwith a weapon typically creates a much tighter pattern. The formercondition can have a spread of 5 or more dB, while the latter typicallyshows a spread of less than 3 dB. Thus this standard deviation may bevaluable.

The present invention preferably assigns points to a number of suchparameters, although other parameters may be used. The first preferredparameter is the magnitude of the co-polarity return. This in itself isa poor discriminator but it serves as the reference for the othermeasurements. Points are only assigned to this parameter when its valueis extremely large indicating that there is a gross abnormalityassociated with this subject. For example if the co-pol magnitude isgreater than −47 dBm, three points may be assigned; if it is greaterthan −50 dBm, two points may be assigned; and if it is greater than −55dBm, one point may be assigned. The unit dBm is an absolute measure ofthe power relative to one milliwatt. The second preferred parameter isthe magnitude of the complex cross-polarity return. For example, if theX-pol magnitude is greater than −60 dBm, 2 points may be assigned, andif it is greater than −62 dBm, 1 point may be assigned. The thirdpreferred parameter is the resulting difference between the magnitudesof the co-polarity and complex cross polarity returns. For example, ifthe difference is less than 5 dB, four points may be assigned; if thedifference is less than 8 dB, two points may be assigned; and if thedifference is less than 10 dB, one point may be assigned. The fourthpreferred parameter is the time shift between the cross-pol and co-polsignals. A fifth preferred parameter is the shape of the cross-polwaveform; the greater the spread in time of the transformed signal, thegreater the probability that the returns are the result of severalsignificant reflectors on this subject. Depending on the measured valueof these parameters with reference to empirically determined thresholds,each is preferably assigned a number of points.

The points for each preferred parameter are preferably then added, andif the total is more than an arbitrary or statistically determined upperthreshold, it is declared that the person has a weapon or other object;if the total is between a lower threshold and the upper threshold acaution (i.e., retest) is preferably reported, and if the total is lessthan the lower threshold, it is preferably declared that the person issafe. Preferably two successive “cautions” results in a decision for aweapon or object.

A running total of preferably three successive “snapshots” of the targetis preferably performed. The snapshots are preferably taken in about ¼second increments. Thus, a set of “snapshots” is preferably completed inless than 3 seconds and is taken in slightly different positions as theperson moves through the range gate. This is much more meaningful as aweapon may be missed in one position and detected in the others.Alternatively, the target may optionally be asked to rotate a certainamount, for example 120 degrees, with readings taken at each position.Or, more than one apparatus may be placed in different positions andpreferably illuminate the target simultaneously from differentorientations. If any one of the three snapshots determines that there isa weapon, then preferably the declaration is that there is a weapon. Ithas however, been found that if the three successive snapshot points aretotaled, there is a significant increase in the accuracy of thedeclaration. Optionally the declaration of a weapon may be determinedaccording to the criteria that each of the snapshots has a minimumnumber of points for three successive snapshots.

The above process describes a manual implementation of a patternrecognition system. The points assigned to each parameter and theirvariation with values of those parameters are manually assigned as aresult of subjective human decisions. The next step in achieving a moreaccurate method of determining the weighting of the parameter values(point assignments) is to use an artificial intelligence or a patternrecognition technique, preferably artificial neural net processing. Thepresent invention utilizes software entitled “Pattern RecognitionWorkbench” (PRW), but any similar such software can be applied. Theprogram is trained by entering the data plus the correct answer; theprogram then evaluates the data and determines the optimal weighting ofeach parameter to maximize accuracy. With a small sample of targets (forexample, people) a data set can be 100% correct. The output is computercode which is stored in the computer that will do the actual processing.The internal computer then operates in a “hands off” mode to renderdecisions on any new data accumulated. When applied here, the resultsare astounding and have improved the prediction accuracy from about 80%for the manually selected weighting to better than 98% for theartificial intelligence selected weighting. This is because the neuralnetwork learns when there is an error; for example, when more people ofvarying sizes and shapes are tested and some do not fit the existingpattern.

A flowchart describing operation of a preferred embodiment of thepresent invention is presented in FIG. 11.

The system is preferably set up to output a “weapon/no weapon” or“object/no object” decision. The system may optionally provide to theoperator a score or ranking which is a measure of the certainty of thedecision. This allows the operator to apply his or her own judgment tothe situation (for example, taking into account the appearance of thesubject). To this end the system of the present invention preferablycomprises a video camera and a laser. The video camera permits theoperator to be remotely located, in a safe area, and observe the subjectbeing examined. In addition, the subject's score and the neural netdecision are preferably shown as an overlay on the video picture.Furthermore, the units preferably accommodate a computer keyboard orsimilar device so that the operator can start and stop the unit'soperation. The operator preferably may change system sensitivity via thekeyboard and select front or rear detection in those situations wherethe subject needs to turn around.

The laser is preferably used to assist in aiming the units during systeminstallation and setup. The laser will also preferably be operated viathe keyboard so that the operator can ensure that the system is properlyoriented, and to check who is being examined. The laser spot preferablyshows up on the video screen.

The system also preferably comprises a second video output to permit asecond TV screen to be used to prompt the subject. Upon detection of asubject in the system range gate, the instructions to the subject appearon the prompter screen. In its quiescent state, the instructions maysimply read ENTER TEST AREA or an equivalent message. When a subjectenters, the screen instructs him or her to, for example, extend theirarms straight out, and then informs the subject when the test iscomplete. This screen preferably does not show the results of the test,which is displayed only on the remote operator's screen.

The system also preferably comprises a simple contact closure outputthat can be used to trigger any device, such as an alarm or door lock.For example, a very small indicator box that has a red and green lightand an audible beeper may be triggered. This type of box can alsofunction as a simple decision output. However, the video output is muchmore informative for the operator and also provides feedback forkeyboard operation.

In addition to detecting concealed weapons, the present invention mayalternatively be employed to detect and/or locate any type or number ofobjects, including but not limited to distinctly shaped merchandise,manufacturing products or work in process, or inexpensive tags attachedto merchandise or other items as an inventory control and/oranti-shoplifting system. The invention may optionally be incorporatedinto an automatic door control system, for example one comprisingautomatic door opening, closing, or locking equipment. The system mayalso be employed as a bomb or explosive detection device. Detection ofother objects is a matter of determining which band of frequencies wouldyield maximum information for the object in question and then accumulatesufficient data to train the artificial neural net.

Continuous Wave Operation

A preferred embodiment of the present invention uses continuous wave, orCW, radar, rather than pulsed radar. There are a number of advantagesfor CW operation. It is less expensive to produce, since the circuitcomplexity is lower, and with a pulsed system components must be usedwhich are fast enough to generate the narrow pulses required for shortrange radar. In addition, accuracy of the phase measurements issignificantly increased, and the time-domain output waveforms resultingfrom the Chirp-Z transform are more uniform, stable, and accurate.

FIG. 12 presents a non-limiting example of a schematic block diagram ofcircuitry for implementing a preferred CW embodiment of the invention.Low power radio transmitter 112 is coupled via first directional coupler113 to switch 120 and transmitter output amplifier 118, which isconnected to transmit antenna 181 through band pass filter 116. Since inCW operation signal transmission and reception are preferably performedsimultaneously, separate antennas are preferably used for each function.There is no longer a need for a transmit/receive switch or a range gateswitch, since range gating is preferably performed mathematically viasoftware processing, as discussed below. Co-pol antenna 180 andcross-pol antenna 182, which detects energy at an orthogonalpolarization, collect energy reflected back from the target.Alternatively, a single dual-polarized antenna may optionally be used.Polarity selection switch 124 in the receive path selects either thehorizontally or vertically polarized antenna or port and determineswhich signal is fed to the receiver at any given time. The input signalis then fed to mixer 132 through band pass filter 136 and receivelow-noise amplifier 134. The output of phase locked local oscillator 130is also fed to mixer 132 via second directional coupler 111.

Intermediate frequency gain control amplifier 140 receives the mainsignal input from mixer 132 through band pass filter 141. The outputfrom amplifier 140 then passes to power divider 144, which splits thesignal into two outputs. One output is amplitude demodulated in detector146 and then digitized in analog-to-digital converter 168 before beingfed back to processor 126. The second output from power divider 144 isfed to phase detector 165 to measure the phase shift of the returnedsignal. The reference phase of a sample of the transmit signal is takenfrom transmitter 112 through directional coupler 113 and fed to mixer109 along with a sample of the local oscillator signal taken from localoscillator 130 through directional coupler 111. The output of mixer 109is fed to amplifier 107 and then to phase detector 165. The output ofphase detector 165 is analog and subsequently digitized inanalog-to-digital converter 168. The digitized signals are fed tocontroller 126 for subsequent processing.

The spread spectrum radar of the present invention is preferablyoperated such that a first frequency is selected for operation for apredetermined time interval and the magnitude and phase of the reflectedsignal at that frequency are then preferably measured. The process isrepeated for other frequencies until the entire desired frequency rangeis covered. A frequency domain waveform is thereby generated. A Hammingfilter is preferably used to select the desired portion of thatwaveform. The Hamming window preferably scales the data by apredetermined factor to reduce ringing caused by the finite number ofsampled frequencies. The resulting frequency domain pattern ispreferably processed by a Chirp-Z transform, which creates a range-gatedtime domain signal; that is, the output signal spans a specified timerange. Since the time range correlates with a specific range ofdistances to the target, the system provides the ability to selectsolely that portion of the return that is reflected from the target,i.e. at the desired distance from the apparatus, just as if a physicalrange gate switch had been used. Signals from other objects that arriveat different times are preferably ignored. Thus with mathematical rangegating it is easier for the operator to both set the distance when theunit is installed as well as change the desired distance if, forexample, the unit is moved at a later time. Conversion from logarithmicto scalar values before Hamming filtering, and conversion back again tologarithmic values after the Chirp-Z transform, may optionally beperformed. Processing of the signals through the neural network or othercircuit then preferably proceeds generally as described above.

Multiple Unit Operation

More than one detector of the present invention may be usedapproximately simultaneously, or in sequence but within a very shorttime frame (e.g. milliseconds), on the same target to improve objectdetection capabilities. For example, units may be mounted in differentpositions surrounding a target, offering different viewpoints so aweapon or other object that is concealed on the side of the targetopposite from one of the detectors is exposed to a different detector.For example, multiple units may be placed at the same height ordifferent heights but looking at the subject from different azimuthalangles. Such multiple detectors may render decisions independently ofeach other.

In addition, the accuracy of detection can be greatly enhanced by theuse of two or more units sharing various data among them. In oneembodiment, each unit operates independently in terms of transmittingand receiving a signal, and then the units share each of their storedco-pol signal amplitudes (for example). In another embodiment, one ormore units collect the co-pol signal amplitude data resulting from theirown transmissions as well as transmissions from the other units. Anaverage co-pol can then be determined and used to provide a moreaccurate reference for the other readings. Each unit can then comparethe cross-pol amplitude to not only its own co-pol reading but to theaverage of all the co-pol readings. Averaging of the cross-pol signals,as well as using other parameters, has also been found to be useful. Ina non-limiting example, the number of parameters that have typicallybeen found useful in detecting concealed weapons using two units issixteen, as opposed to typically only six from a single unit. Each ofthese parameters preferably shows a positive trend, and as long as eachis more than 50% accurate, combining the results of so many parameterssignificantly increases the accuracy of the system and robustness of thedecision. Another possible example of a parameter includes polling toimprove the accuracy of the co-pol reference to measuring small timedifferences between reflections.

When multiple units are used, only one unit should be transmitting atany given time. Thus the CW operation of each unit is preferablyinterrupted. This is not the same as pulsing the radar, since the periodof time that each unit is on is very long compared to the transit timeof a signal from the radar to the target and back again, and is alsovery long to the measurement circuitry's time constants. The systempreferably changes frequency approximately every 27 microseconds, so ifthree units are used in a system (for example, to enable viewing atarget from all azimuths simultaneously), each unit has approximatelysix or seven microseconds to take a reading at each frequency. This timeis a sufficiently long period. This interlacing allows readings fromthree units to be obtained in the same amount of time that other systemsneed to obtain the readings from one unit.

The software preferably comprises an easy to use graphical userinterface (GUI). The installer can preferably easily select the numberof units in a system, check that all units are communicating, andactually view the time domain plots. This can help the operatordetermine if there is any hidden clutter, such as rebar in the floor ormetal studs in the walls, that might interfere with system operation.The GUI also preferably allows the operator to select decisionthresholds in order to set the system's sensitivity.

EXAMPLE 1

Targets which were tested for detection include a variety of weapons,including a .22 caliber pistol, a Glock 9 mm semiautomatic pistol, anUzi assault rifle, and a variety of terrorist style bombs includingthose comprising nails as shrapnel, slingshot balls as shrapnel andexplosive simulation packets without shrapnel. Table 2 displays sampletest data regarding the detection of a variety of weapons in accordancewith the present invention using the 9.5 to 10.7 GHz frequency band andilluminated with horizontal polarization. Data were taken for frontviews only. This data was taken using 12 different people whose size andweight spanned from about 100 pounds to 220 pounds and heights fromabout 5′0″ to 6′2″. As shown, the system was correct for 115 out of 116safe cases thereby yielding only one false positive and zero falsenegatives out of 283 weapon cases. (NA means not applicable.)

TABLE 2 Number Cor- % False False Case of Tests rect Correct NegativesPositives Cautions .22 Pistol 60 60 100% 0 NA 0 Balls 59 59 100% 0 NA 0Glock 57 57 100% 0 NA 0 Nails 62 62 100% 0 NA 0 Uzi 45 45 100% 0 NA 0 No116 115 99% NA 1 0 Weapon

Although the present invention has been described in detail withreference to particular preferred and alternative embodiments of theinvention, other embodiments can achieve the same result. Personspossessing ordinary skill in the art to which this invention pertainswill appreciate that various modifications and enhancements may be madewithout departing from the spirit and scope of the claims that follow.Variations and modifications of the present invention will be obvious tothose skilled in the art and it is intended to cover all suchmodifications and equivalents. The circuit components that have beendisclosed above are intended to educate the reader about particularpreferred and alternative embodiments, and are not intended to constrainthe limits of the invention or the scope of the Claims. Although thepreferred embodiments have been described with particular emphasis onspecific hardware configurations or frequency bands, the presentinvention may be implemented using a variety of circuit components orfrequency ranges. Although specific signal processing methods andapparatus have been described with particular emphasis on Chirp-ZTransforms, the alternative embodiments of the present invention mayalso be implemented using a variety of other mathematical methods. Theentire disclosures of all patents and publications cited above arehereby incorporated by reference.

1. A method for detecting an object concealed on a target, the methodcomprising the steps of: transmitting continuous wave electromagneticradiation to the target at selected frequencies within a frequencyrange; receiving two orthogonally polarized signals reflected from thetarget at each selected frequency; measuring the amplitude and phase ofthe reflected signals; generating a frequency domain waveform;transforming the waveform to a time domain in a time windowcorresponding to a distance to the target; and processing the timedomain waveform to determine whether an object is concealed on thetarget.
 2. The method of claim 1 further comprising the step offiltering the frequency domain waveform.
 3. The method of claim 2wherein the filtering step comprises using a Hamming filter.
 4. Themethod of claim 1 wherein the transforming step comprises using aChirp-Z transform.
 5. The method of claim 1 wherein the transformingstep results in range gating of the signal.
 6. The method of claim 1employing a plurality of units, each unit performing the transmittingand receiving steps.
 7. The method of claim 6 wherein the units performthe transmitting step sequentially.
 8. The method of claim 7 whereincontinuous wave transmission of each unit is interrupted to allow theother units to transmit radiation.
 9. The method of claim 8 wherein thetime of continuous wave transmission of each unit is long compared tothe transit time of the radiation from the unit to the target and backto the unit.
 10. The method of claim 6 wherein the units perform thereceiving step sequentially.
 11. The method of claim 10 wherein eachunit receives signals transmitted solely by its own transmitter.
 12. Themethod of claim 6 wherein each unit receives signals transmitted by atransmitter of another unit.
 13. The method of claim 6 wherein reflectedsignals received by the units are compared.
 14. The method of claim 6wherein signal amplitudes received by each unit are averaged.
 15. Themethod of claim 6 wherein the units are disposed at different heightsrelative to the target.
 16. The method of claim 6 wherein the units areoriented to illuminate the target from different viewpoints.
 17. Themethod of claim 1 wherein the object is selected from the groupconsisting of weapon, firearm, bomb, suicide vest, explosive,merchandise, tag, work in process, inventory, and manufacturing product.18. The method of claim 1 performed to prevent shoplifting or inventorytheft.